The present invention relates to a coupling lens forming an optical system of a device, such as an optical information recording/reproducing device for recording information to and/or reproducing information from an optical disc.
In general, an optical information recording/reproducing device is configured such that the degree of divergence of a light beam emitted by a light source is converted by a coupling lens and the converted light beam is converged by an objective lens onto a record surface of an optical disc, such as a CD or a DVD, being used. Incidentally, in this specification, the “optical information recording/reproducing devices” include devices for both information reproducing and information recording, devices exclusively for information reproducing, and devices exclusively for information recording. In the following, the term “compatibility” is used to mean that the optical information recording/reproducing device ensures the information reproducing and/or information recording with no need of component replacement even when the optical disc being used is switched.
In most cases, lenses of the above described types provided in optical information recording/reproducing devices are made of resin. Therefore, the refractive index of such a lens varies depending on change in environmental conditions, such as temperature variations. More specifically, the actual refractive index may change from a design refractive index defined when the lens is designed. The change in refractive index of a lens leads to change of a positional relationship between an object point and an image point with respect to the position of the lens.
It is noted that a lens made of resin has a linear expansion coefficient larger than that of a lens made of glass. Such a property of a lens made of resin causes a considerable amount of change of the positional relationship between an object point and an image point with respect to the position of the lens.
Even if change of the positional relationship between an object point and an image point with respect to the position of the objective lens is caused due to change of the refractive index of the objective lens, the optical information recording/reproducing device is able to cancel the effect of the change in refractive index by executing a focusing function which is one of primary functions of the optical information recording/reproducing device.
However, if change in refractive index of the coupling lens which is placed only on an approaching optical path of a light beam proceeding to an optical disc (but is not placed on a returning optical path of the light beam returning from the optical disc) occurs, the following problems occur.
First, the light beam emerging from the coupling lens may have an unintended degree of divergence. If the degree of divergence of the light beam emerging from the coupling lens changes, the spherical aberration is caused. In this case, it becomes difficult to form a suitable beam spot for information recording or information reproducing on a record surface of the optical disc. Second, since typically a photoreceptor for receiving light reflected from the record surface of the optical disc is placed at a position having a conjugate relationship with an image point of the coupling lens, the conjugate relationship is lost if the image point of the coupling lens changes due to change in refractive index or linear expansion and thereby it may become difficult for the photoreceptor to execute an accurate detecting operation.
Examples of configurations for solving the above described problems are discussed in Japanese Patent Provisional Publications Nos. 2001-159731A (hereafter, referred to as JP2001-159731A) and 2003-114382A (hereafter, referred to as JP 2003-114382A).
The coupling lens disclosed in JP2001-159731A is provided with a diffraction structure so that the problem that the degree of divergence of a light beam emerging from a coupling lens changes due to change in refractive index can be solved. The coupling lens disclosed in JP 2003-114382A is also provided with a diffraction structure so that the problem that an image point relative to an object point changes due to change in refractive index can be solved. However, it is noted that the diffraction structure increases the manufacturing difficulty of the coupling lens and thereby increases the manufacturing cost of the coupling lens.